The invention is an improvement in a liquid-metal dip seal for a liquid-metal-cooled fast-breeder nuclear reactor which renders unnecessary a gas pressure equalization system.
Liquid-metal-cooled fast-breeder nuclear reactors ordinarily comprise a reactor core submerged in a pool of liquid-metal coolant contained in a reactor vessel. The remaining volume inside the vessel is occupied by a cover gas. The reactor vessel is closed at the top by a vessel head. See, for example, J. R. Dietrich and W. H. Zinn Ed., Solid Fuel Reactors, Addison-Wesley Publishing Company, 1958, at page 157. The vessel head contains one or more rotating plugs each nested within the next larger plug or the vessel head itself. The purpose of the rotating plugs is to position refueling tools anywhere over the reactor core. Since it is essential to prevent out-leakage of highly radioactive cover gas, the annulus between two rotating plugs, or a plug and the reactor vessel head, must be effectively sealed, both when the plugs are stationary or moving with respect to each other.
A liquid-metal dip seal is used in order to provide effective sealing between two nested rotating plugs while still permitting relative rotation between the plugs. The larger of two nested plugs has a trough located on its inner periphery; the trough is similar to a gutter and is comprised of a flange extending from the inner periphery of the larger plug, which flange serves as a trough bottom, and a side wall fixed at a right angle to the edge of the flange away from the plug. The periphery of the larger plug serves as the other side wall of the trough and the height of the side walls are determined by the desired depth of liquid metal to be contained in the trough.
The inner and smaller rotating plug has sections with two different diameters, the larger diameter section being above the smaller diameter section. A dip seal blade is pendently supported from the larger diameter section; the blade extends completely around the periphery of the smaller rotating plug. When both plugs are nested together, the dip seal blade extends into the trough on the larger rotating plug and divides the trough into two dip seal legs. The blade does not touch the bottom of the trough and therefore the legs communicate underneath the dip seal blade. The trough is then filled with liquid metal which prevents cover gas from passing underneath the seal blade and thus escaping from the reactor vessel.
The hydrostatic pressure of the seal, controlled by the depth of the liquid metal in the trough, is capable of resisting small pressure transients in the cover gas. In order to prevent excessive cover gas pressure from forcing the liquid metal in the seal out of the trough, the portion of the plug annulus above the dip seal is frequently connected via a gas pressurization system to the cover gas so that the pressure of the plug annulus above the dip seal is matched to that of the cover gas, thereby preventing excessive pressure differences on the dip seal which would overcome the hydrostatic pressure imposed by the dip seal. A cover gas pressure transient arises from, for instance, a reactor scram, which causes a contraction of the liquid metal coolant and consequent expansion of the cover gas. A cover gas makeup system reduces the magnitude of, but does not eliminate, the cover gas pressure drop as well as overshoot on the restoration of normal pressure. See, for example, Thorel et al., U.S. Pat. No. 3,819,478, issued June 25, 1974. Thorel points out at column 6, lines 64-68, and column 7, lines 1-21, that the gas pressure equalization system is an alternative to making the seals themselves deep enough to withstand reactor cover gas pressure variations.
The use of such a gas pressure equalization system has several disadvantages. First, it decreases reliability because of its added complexity and active function. Second, failure of the gas pressure equalization system to respond properly to a cover gas pressure rise would cause radioactive cover gas to bubble under the dip seal blade into the annulus above the dip seal which can result in radiation dose rates of several roentgen per hour for anyone in the vicinity of the reactor vessel head. Last, the inert gas introduced to and removed from the portion of the annulus above the dip seal may contain oxygen as a contaminant. This oxygen reacts with the liquid metal in the dip seal and forms oxides which eventually interfere with plug rotation and seal function.